Melting Point Of Oraganic Compound

Experiment 7

Title : Melting Point Of Organic Compound

Objective :

To identify the unknown organic compound by melting point and depression method
To practise using melting point apparatus to measure the melting point

Introduction :

The melting point of a solid is the temperature range at which it changes from solid to . At the melting point the solid and liquid phase exists in equilibrium. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point. Because of the ability of some substances to , the freezing point is not considered to be a characteristic property of a substance. When the "characteristic freezing point" of a substance is determined, in fact the actual methodology is almost always "the principle of observing the disappearance rather than the formation of ice"

The melting point of an organic compound can be determined by introducing a tiny amount of the substance into a small capillary tube, attaching this to the stem of a thermometer centered in a heating bath, heating the bath slowly, and observing the temperatures at which melting begins and is complete. Pure samples usually have sharp melting points, for example 149.5 ºC – 150 ºC or 189 ºC – 190 ºC; impure samples of the same compounds melt at lower temperatures and over a wider range, for example 145 ºC – 148 ºC or 187 ºC- 189 ºC. The contaminant that depresses the melting point and extends the melting range may be an indefinitely characterized resinous material or it may be a trace of a second chemical entity of melting point either higher or lower than that of the major component. Under equilibrium conditions (no super-cooling) the temperature at which a pure solid melts is identical with that at which the molten substance solidifies or freezes. Just as salt lowers the freezing point of water, so no one compound (A) depresses the melting point of another (B) with which it is mixed. If pure A melts at 150 ºC-151 ºC and pure B at 120 ºC-121 ºC, mixture of A with small amount of B will melt unsharply at temperatures below 150 ºC and mixtures of B containing A will melt below 120 ºC. Both the temperature and sharpness of melting point are useful criteria of purity.

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A third substance C may have exactly the same melting points as A, namely 150 ºC-151 ºC, but if a mixed melting point determination is made, that is if A and C are mixed and the melting point of the mixture is observed, the one substance will be found to depress the melting point of the other. Depression of melting point or non-depression is invaluable in the identification of unknowns.

An unknown D found to melt at 150 ºC -151 ºC can be suspected of being identical with one or ...

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An unknown D found to melt at 150 ºC -151 ºC can be suspected of being identical with one or the other known substance A and C; observation that the mixture A and D shows a melting point depression would exclude identity with A, failure of C to depression the melting point of D would prove C and D identical. If a substance melts at 150 ºC or higher, the thread of mercury in the upper part of the thermometer is cooler that that in the bulb and hence the temperature recorded is a little lower than the actual bath temperature. The extent of the error due to the stem exposure depends upon the design of the thermometer and the type of heating bath used; it may amount to 2 ºC to 5 ºC at 200 ºC , 3 ºC-10 ºC at 250 ºC. An approximate correction for stem exposure can be calculated from the formula :

Stem correction (ºC= 0.000154 (t-t’) N)

Where the fraction represents the difference in the coefficients of expansion of glass and of mercury, t is the temperature read, t’ is the average temperature of the exposed column of mercury (determined, approximately , by reading the temperature of a second thermometer whose bulb is placed midway between the bath and the point on the first thermometer corresponding to t), and N represents the length, measured in degrees, of the thread exposed between the top of the heating liquid and the point t. the convention for reporting a corrected melting point is: m.p 283.5 ºC- 284.5 ºC (corr). The error can be eliminated by use of a heating bath designed to accommodate a series of short thermometers that can be totally immersed.

Apparatus and Materials:

Ice
Benzoic acid
Urea
Unknown compound (X)
Thermometer

Procedures :

Part 1: Melting points of standard compounds

The bulb of the thermometer was inserted into a beaker containing ice slurry. The observed melting point and the corrected melting points of the standard compounds listed were recorded.
A correction chart was constructed by plotting the observed melting points against the corrected melting point. The resulting points were connected and the calibration curve of the thermometer will be observed.

Part 2: Melting points of unknown compounds and mixture

The capillary tube was inverted and the sample of benzoic acid was placed in by knocking the open end of capillary tube to the sample.
The capillary tube was placed into the melting point apparatus (MPA), temperature was set and the start button was pressed.
The start button was pressed again when the plateau light was on.
The sample was observed until the sample has fully melted.
The melting point of benzoic acid was recorded.
Step 1-5 was repeated using urea, 10 mol per cent benzoic acid and 90 mol per cent urea and 90 mol per cent benzoic acid and 10 mol per cent urea as samples.

Part 3: Identification of an unknown compound by the mixed melting point method

Samples A, B, C, D, E and F were obtained.
The melting point of each sample was determined by repeating steps 1-5 with each of the samples. The melting points of each sample were recorded.
The identity of each of the samples was determined by identifying their melting points.

Results :

Melting point of ice : 0 ºC-0.1 ºC

Table 1: The results for part 2 in determining the melting point of known sample

Table 2: The results for part 3 in determining the melting point of unknown sample

Table 3: Identification of the unknown substances

Discussion :

Crystalline solids are composed of atoms, ions, or molecules in a highly ordered geometric pattern (the crystal lattice). These atoms, ions or molecules are held in their positions by electrostatic, dipole and Van der Waals forces. When a pure crystalline solid is heated, the atoms, ions, or molecules begin to vibrate more and more rapidly until at a definite temperature the thermal motion of the particles is great enough to overcome the forces of attraction. Then they enter a more random and mobile state, the liquid state. The melting point of a solid is defined as the temperature at which the solid and liquid phases of a substance are in equilibrium. The freezing point of a liquid is the same temperature as the melting point of its solid. However, freezing points are rarely measured in practice because solidification may not occur at the correct temperature due to the phenomenon of supercooling.

Determination of the temperature at which the solid and its liquid phases of a substance are in equilibrium is tedious and time consuming; it is also quite difficult to do if there is only a small amount of sample available. Thus, in practice, most melting points are determined as capillary melting points, which can be done quickly with a small amount of sample. A capillary melting point is the temperature range over which a small amount of solid in a thin-walled capillary tube begins to visibly soften (first drop of liquid) and then completely liquefies. Melting points recorded in chemical references are capillary melting points unless otherwise stated.

A solid is said to melt sharply if the melting point range is 1-2°C. A pure solid will generally melt sharply because the forces of attraction between its particles are the same. However, the presence of a foreign particle interrupts its uniform structure and the attractive forces are weakened. An impure solid melts at a lower temperature and over a wider range than a pure solid. Thus, the melting point of a solid not only helps to identify the compound but also gives an indication of purity.

Suppose that two unknown compounds A and B have identical melting points and appear to be identical. One can easily determine whether or not A and B are really the same compound by determining the melting point of a mixture of A and B. (The melting point of a mixture is called a mixture melting point). If A and B are the same substance, the mixture melting point will be same. However, if they are different, one compound will act as an impurity in the other and the mixture melting point will be lower and wider than the melting point of pure A or pure B.

It should be noted, however, that there is one unique mixture of two compounds A and B that has a lower melting point than the mixture of any composition of the two. That particular mixture is called the eutectic mixture. The melting point of the eutectic mixture is called the eutectic point. A mixture whose composition corresponds exactly to its eutectic mixture will have a relatively sharp melting point. Thus, there is a possibility that an eutectic mixture could be mistaken for a pure compound. However, if a small amount of either A or B is added to the mixture (assuming they are both known), the melting point of the resulting mixture will be higher and more spread out than the melting point of the eutectic mixture.

In this experiment, a table of compounds with their respective melting points was given. We were required to determine the melting point of the unknown substances A, B, C, D, E and F and to identify their identity by comparing their melting point to the melting point of compounds given in the table. We have successfully identified the substances A, B, C, D, E and F as benzoic acid, o-benzoylbenzoic acid, salicylic acid, acetylsalicylic acid,benzoin and anthranilic acid respectively.

The observed melting points of the some of the sample compounds do not agree exactly with the recorded or given melting points in the table. This is because the samples provided might not be pure and contain impurities such as dust and debris collected from the surrounding. The presence of impurities will give a slightly lower or higher melting point and the melting point will be in a wider range. The other cause is that an error can be made by us when we observe the melting point. We might have missed the temperature of the first drop of liquid formed and take the temperature of the compound when half or most of the sample compound have liquidified.

Conlusion :

We successfully identified the melting points of known organic compounds using the melting point apparatus, and we also successfully identified the unknown compounds A, B, C, D, E and F as benzoic acid, o-benzoylbenzoic acid, salicylic acid, acetylsalicylic acid,benzoin and anthranilic acid respectively, by identifying their melting points.

Reference :